Concentrating solar power (CSP) uses mirrors to focus solar energy to boil water and to make high pressure steam. The steam subsequently drives a turbine and generator unit to generate electricity. There is a need to bring CSP electricity cost down to the point of being competitive with traditional fossil fuel-based electricity. An advanced low melting point heat transfer fluid (HTF) with a high thermal stability is a key technical advance necessary to reduce the cost of CSP electricity. Such a material would enable higher temperature operation and increased efficiency in converting solar energy to electricity. Increasing the maximum fluid output temperature of current CSP plants from 390° C. to 500° C. would increase the conversion efficiency of the Rankine power block, thereby reducing the levelized energy cost by 2 cents/kWh. Achieving 500° C. operation would also double the effectiveness of sensible heat thermal storage systems, significantly reducing the capital cost of thermal storage (Justin W. Raade and David Padowitz, Development of Molten Salt Heat Transfer Fluid With Low Melting Point and High Thermal Stability, J. Sol. Energy Eng. 133, 031013 (2011)).
Furthermore, an advanced low melting point heat transfer fluid could also be used as a thermal energy storage fluids in solar energy applications. Heat storage allows a solar thermal plant to produce electricity at night and on overcast days. This allows the use of solar power for continuous power generation as well as peak power generation, with the potential of displacing both coal- and natural gas-fired power plants. Additionally, the utilization of the generator is higher which reduces cost.
Heat is transferred to a thermal storage medium in an insulated reservoir during the day, and withdrawn for power generation at night. The envisioned product should be cheap to produce, easy to make and easy to handle.
A number of different solutions to the aforementioned problem has been proposed, one of them being the use of nitrate-based salts used as a melt (molten salt). Molten salts exhibit many desirable heat transfer qualities at high temperatures. They have high density, high heat capacity, high thermal stability, and very low vapour pressure even at elevated temperatures. Their viscosity is low enough for sufficient pumpability at high temperatures, and many are compatible with common stainless steels. Salts of many varieties are currently available in large commercial quantities from several suppliers.
Most commonly, a binary salt based on NaNO3 and KNO3 and commonly known as Solar Salt, is used as heat transfer fluids and as thermal energy storage fluids. The Solar Salt consists of the eutectic mixture of 60% NaNO3 and 40% of KNO3. NaNO3 melts at 307° C. and KNO3 melts at 337° C. The mixture at its eutectic point exhibits a drastically reduced melting point of 222° C. This represents a melting point suppression of 85° C. from the lowest melting single component. It is produced as a double salt in solid (granular) form, for example as disclosed in U.S. Pat. No. 4,430,241(Fiorucci, 1984). Although the salt is very cheap and has a high thermal stability, a major drawback of this salt is its high melting temperature (220° C.).
Bradshaw et al. in Solar Energy Materials 21 (1990) 51-60 describe the use of ternary nitrate salts comprising nitrate salts of Na, Ca and K, for solar thermal energy systems (see Table1 therein). Adding Ca(NO3)2 to the Solar Salt showed to lower the melting temperature, which is an advantage as it lowers the risk of solidifying of the salt mixture in the system, blocking pumps, piping, etc.
As is commonly known, Ca(NO3)2 in its anhydrous form is a hygroscopic solid, forming the (liquid) tetrahydrate salt Ca(NO3)2.4H2O with a melting point of about 43° C. It is commercially available as a liquid solution or in solid particulate form, where it is mixed with ammonium nitrate to reduce its tendency to absorb water, in particulate water from the air. In its particulate form, it is difficult to handle.
From the disclosed melt compositions in Bradshaw et al. in Solar Energy Materials 21 (1990) 51-60, it can be concluded that the thermal stability decreases with increasing amounts of Ca(NO3)2. At amounts of 42 weight % of Ca(NO3)2, the decomposition temperature is about 500° C. and a solid phase (CaCO3) was visually detected. Similar research was conducted by the US Department of Energy and the Sandia National Laboratories and several reports are available on the internet (see e.g. Steven St. Laurent, Thermocline Thermal Storage Test for Large-scale Solar Thermal Power Plants), where a melt mixture is disclosed manufactured from melting together 30 weight % of Ca(NO3)2, 24 weight % of NaNO3, and 46% of KNO3, all in solid form.
Hitec XL (Coastal Chemical) is commercially available as an aqueous solution of a ternary nitrate salt mixture containing 59 weight % water, comprising Ca(NO3)2, NaNO3 and KNO3, of which different compositions have been reported. When the water is boiled off, the melt has a composition which has been reported to be 15% NaNO3, 43% KNO3 and 42% Ca(NO3)2 (Kelly et al., 2007), whereas Kearney et al (2003) quote 7% NaNO3, 45% KNO3 and 48% Ca(NO3)2. The eutectic mixture lies at a concentration ratio of 7%/30%/63% (ENEA, 2001). In practice, and mainly for cost reasons, the exact eutectic concentration is not employed as the solidification temperature is not very sensitive to the exact mixing ratio (Large-Scale Solar Thermal Power, Werner Vogel and Henry Kalb, Wiley-VCH Verlag GmbH & Co, KGaA, Weinheim, 2010, page 245. The Hitec XL mixture is manufactured by dissolving the three salts (Ca(NO3)2, NaNO3 and KNO3) in water. It has the disadvantage that, in order to obtain the eutectic melt, this high amount of water (59 weight %) needs to be boiled off, leading a large energy consumption.
U.S. Pat. No. 7,588,694 B1 (Bradshaw et al., 2009) describes the use of quaternary compositions comprising nitrate salts of Na, K, Li and Ca. For comparison, an eutectic nitrate salt composition is disclosed containing 21 mol % Na, 49 mol % K, and 30 mol % Ca, having a melting temperature of 133° C. (Table 2). No decomposition temperature is given. Also, for comparison, an eutectic nitrate salt composition is disclosed containing 30 mol % Na, 50 mol % K, and 20 mol % Ca, having a melting temperature of 505° C. (Table 3). No melting temperature is given. The use of lithium nitrate is undesirable due to the high cost thereof.
There is little or no research on higher order mixtures of nitrates.
Hence, there is a need for a low-cost nitrate salt based mixture that has both a low melting temperature and a high decomposition temperature, and is easy to make and to handle.